The present invention pertains generally to thermoplastic polymer blends comprising a monovinylidene aromatic copolymer in combination with an acetal polymer and an elastomeric polymer selected from the group consisting of thermoplastic polyurethanes, copolyester elastomers and combinations thereof. In one particularly preferred embodiment hereof, the monovinylidene aromatic copolymer is a rubber-modified monovinylidene aromatic copolymer having from about 1 to about 40 weight percent of dispersed rubber particles contained therein.
Blends of various and varying types of polymeric materials have been suggested over the years in a variety of prior art references. Of these, the prior art references utilizing acetal resins as blend components generally illustrate compositions in which the acetal resin forms the major or continuous phase, presumably because of large shrinkage and associated interfacial stress build-up occurring upon cooling from the melt. For example, U.S. Pat. No. 4,665,126 to Kusumgar et al. discloses certain polymeric molding compositions containing a predominant amount (e.g., from about 60 to 95 weight percent) of an acetal polymer ingredient in combination with relatively lesser amounts (e.g., from about 4 to 30 and from about 1 to 10 weight percent, respectively) of a thermoplastic polyurethane (TPU) and a multiphase composite interpolymer such as, for example, a butadiene-based, rubber-modified styrene/methyl- methacrylate polymer. Such Kusumgar et al. formulations are said to have improved impact strength relative to that of the acetal polymer per se and relative to that of comparable two component acetal/TPU or acetal/multiphase composite interpolymer blends and to be useful in various molding applications.
U.S. Pat. No.. 4,694,042 to McKee et al. pertains to thermoplastic molding polymer blends containing a minor proportion (i.e., from 5 to 50 parts) by volume of a partially or completely crystalline polymer such as nylon, polyacetal, etc. wherein said crystalline polymer, even though employed in minor volumetric proportion, is nevertheless considered to form a coherent phase and wherein the second, major proportion component forms a dispersed phase therein. Within the indicated McKee et al. blends, said major proportion (i.e., from 50 to 95 parts by volume) component consists of one or more crosslinked, emulsion-polymerized elastomeric polymers such as, for example, butadiene or acrylate rubber-based graft copolymers containing either from 10 to 50 weight percent of a shell having a glass transition temperature of less than -10.degree. C. or a substantially lesser amount of a hard polymer shell of styrene, methylmethacrylate or styrene acrylonitrile copolymer. Acetal resin-based compositions are not evident in the working examples.
British Patent 1,311,305 discloses thermoplastic molding compositions composed of a mixture of from 50 to 99 weight percent of an acetal polymer and from 1 to about 50 weight percent of a butadiene or acrylate rubber-modified, two-phase polymer mixture. Such thermoplastic molding compositions are described as having considerably improved impact strength relative to that of the acetal polymer per se. Preferred embodiments of this reference utilize 80 to 95 weight percent of the acetal polymer component.
U.S. Pat. No. 4,639,488 to Schuette et al. discloses impact resistant polyacetal-based molding materials containing from 30 to 95 weight percent of an acetal polymer and from 5 to 70 weight percent of an emulsion polymerized elastomeric graft copolymer composed, on a graft copolymer weight basis, of from 60 to 90 weight percent of a butadiene-based core (or "grafting base") and from 10 to 40 weight percent of a grafted shell of a styrene and/or methylmethacrylate-based polymer or copolymer. Such molding materials are said to have high impact strength at low temperatures, to exhibit good thermal stability and to resist discoloration in the presence of light.
U.S. Pat. No. 4,179,479 to Carter discloses thermoplastic polymer blend compositions containing from 40 to 100 weight percent of a thermoplastic polyurethane in combination with up to 60 weight percent of a thermoplastic polymer which can be, among a variety of other things, an ABS resin, an acetal resin or a mixture thereof. Such compositions are also required to contain 0.5 to 10 weight percent of an acrylic polymer processing aid in order to improve the processability and molding characteristics of the polymer compositions in question.
U.S. Pat. No. 4,117,033 to Gale discloses polymer blends containing an acetal resin in combination with from 0.1 to 5 weight percent of a low molecular weight copolyether-ester resin. Said copolyether-ester resin is said to improve the melt processability of the indicated acetal resin.
U.S. Pat. No. 4,683,267 to Lindner et al. discloses molding compounds consisting of a mixture of from 60 to 99.00 parts by weight of an acetal resin, from 0 to 40 parts by weight of an elastomer softening below the melting point of said acetal resin and from 0.01 to 40 parts by weight of an aliphatic, rubber-like, high molecular weight adipate-carbonate mixed ester. Elastomers said to be useful in the Lindner et al. blends include homopolymers and copolymers of alpha-defins, homopolymers and copolymers of 1,3-dienes, copolymers and homopolymers of vinyl esters and copolymers and homopolymers of acrylate and methacrylate esters.
Another publication concerned with blends of polyacetal resins and polystyrene resins is Japanese Kokai No 64-38463, published Feb. 8, 1989. Such publication is essentially concerned with polyacetal/polystyrene blends wherein the polyacetal constitutes the major portion by weight thereof and requires in all events that the ratio of the polyacetal melt flow rate (MFR, ASTM D-11238 at 190.degree. C. and 2160g) to the polystyrene melt flow rate (ASTM D-16238 at 200.degree. C. and 5000g) be from 5:1 to 100:1. According to such publication, excellent surface appearance is obtained by operating within, and only by operating within, the indicated range of polyacetal: polystyrene melt flow rate ratios. Also according to such publication, the polymer blends thereof may optionally contain additional polymer ingredients such as a polyurethane resin, olefinic homopolymer or copolymer resins, acrylate resins, polyamide or polyester resins or ABS resins
There remains a continuing need to provide engineering thermoplastic materials having a beneficial balance of processability, good aesthetics with no pearlescence and having alternative, advantageous property profiles such as mechanical strength, impact resistance, creep and chemical resistance and practical toughness as provided by the present invention.